1. Field of the Invention
The present invention relates to an electric steering device, and more particularly to an electric steering device capable of adjusting a tilting position or a telescopic position of a steering wheel by an electric motor.
2. Description of Related Art
Recently, it has been required to adjust the tilting position and telescopic position of a steering wheel according to the physique and driving posture of a driver. For the purpose of facilitating the adjustment of the tilting position or telescopic position using an electric motor, mostly, an electric steering device has been adopted. Such an electric steering device is disclosed in for example PCT Publication WO03/078234.
The electric steering device disclosed in WO03/078234 uses a nut serving as a feeding mechanism which transmits the rotation of an electric motor as the tilting motion or telescopic motion of a column; the nut makes a linear motion by the rotation of a feeding screw shaft.
In order to make compact and improve rigidity of such an electric steering device, it is necessary to make identical the mechanically movable range and the controllably movable range of the tilting motion or telescopic motion wherever possible.
However, where the mechanically movable range and the controllably movable range are made identical wherever possible, if the rotation of the electric motor is not quickly stopped at the moving end of the nut, the nut will strike with a securing portion of the moving end and so tightened by a screw segment of the feeding screw shaft.
When the nut is tightened at the moving end thereof by the screw segment of the feeding screw shaft, since the friction acting on between the nut and the screw segment of the feeding screw shaft is dynamic friction, the friction coefficient is relatively small. When the nut thus tightened is loosened, however, the friction coefficient becomes relatively large because the friction at this time is static friction. Therefore, large torque is required to loosen the nut which has struck with the securing portion and so has been tightened.